a.out =?= executable

[Chris Steinbroner]

according to the as(1) man page, the default output is "a.out."
traditionally, as(1) produces a ".o", which is not an executable,
requiring tweaking with ld(1) to resolve external references, and to
perhaps change the file type in the header to make it executable.

can someone please explain how and why as(1) has changed?  for bonus
points, what is the history behind calling an executable "a.out," even
though ld(1) is what has traditionally produced a.out.

please respond by e-mail, as i don't usually read this newsgroup.